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Light DOMLWC
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Litlight DOM
We also have an audio tag and a button. In order to work with these for playback and event listeners, we need to get a reference to them somehow — calling querySelector() on the shadow root does the trick. We could also have constructed these elements through document.createElement() and appended them manually to the shadow root, retaining references to them for later, but that tends to get messy as the markup gets more complex.
Before web components, UI libraries on the web were a leaky abstraction. They required users to include a stylesheet, carefully engineered so that it wouldn't interfere with other items on the page. To make sure their own styles wouldn't be affected by the page, these stylesheets had to ship an exhaustive and bulky list of extra rules, just in case. A UI element from jQuery or Bootstrap also touched the DOM in unpredictable ways — there was no easy way to exclude the internal markup of its widgets from your own document queries, or be sure that it wouldn't mutate other parts of the page to achieve its goals.
Originally, web components included another API, HTML Imports, that helped developers package their components into a self-sufficient file. The imported HTML would include the script for their behavior, a with their shadow markup, and any other dependencies they might need. Unfortunately, HTML Imports never got broad browser support, which also left in a much less useful place.
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Any element can technically host a shadow DOM fragment, but it's most commonly used for custom elements. You may remember that custom elements are not allowed to alter their contents or attributes in the constructor. Shadow DOM is an exception to this rule. As a result, custom elements will often set up their shadow DOM fragments on creation:
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Keep in mind, the shadow root effectively turns its host into a replaced element, like an or tag. You can place child elements in the light DOM of the host, but they won't show up on the page automatically.
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Let's imagine we're creating a media player for a podcast page as a component. For users of the media player, the element will look something like this:
In the shadow DOM for our element, we've added three tags through a simple HTML block. First, there's a style tag, which applies visual styling to our button. Thanks to the isolation of the shadow DOM, our selectors can be extremely simple, targeting elements only by tag name. Buttons outside the element won't be affected, since the styles can't exit the shadow DOM.
Lwc:dom=manual
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If you query for this element in the browser console, none of this will be visible to you. Users of our can style buttons or run any DOM queries they might choose, and our code won't interfere or be disturbed.
In the end, I don't know that this matters very much. New syntax features like template string literals make it a lot easier to write blocks of multiline HTML in JavaScript itself, instead of cloning it from a node in the page. We can augment our base class to automatically fill the element's shadow DOM:
Style tags are a common way to add CSS to a shadow fragment. It's also possible to include a stylesheet with a tag, but it'll be downloaded and displayed asynchronously, creaing a flash of unstyled content. Most of the time, an inline stylesheet is an easier and simpler solution. Since it's created from JS, and the styles don't leak out into the wider document, it doesn't really create an issue for performance or page weight.
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With modern bundling tools, like Webpack or Rollup, you can write that template as a separate HTML file and import it. From there, you can use the lookup on this.elements to mount a more comprehensive templating solution, like lit-html, onto sections of the shadow if you need to.
This code looks for a static template property on the class definition. If it's there, it splats that string into the shadow root. Any elements in the template with an "as" attribute are made available on the elements property for easy access. This isn't a sophisticated templating system, but it handles a lot of use cases without a lot of code, especially with the other convenience features of our base class. For example, a simple button element:
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attachShadow() has to be called with a mode, which can be either "open" or "closed". If it's open, the element will have a shadowRoot property that can be used to get into it from ouside. Most experts agree that, given the infamously leaky nature of JavaScript, there's not really any security benefit of a closed root, and it makes debugging a lot harder. Default to open, if you can.
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Much of this code should look familiar: we have a method that triggers playback from and event listener, and an attributeChangedCallback that updates our player when the "src" attribute is set.
Light DOMvs ShadowDOM
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Built-in elements don't have this problem, sometimes notoriously so: it's almost impossible, for example, to style a select box no matter how much you (or your designer) want to. That's because they've historically had a capability that wasn't exposed to independent developers: they could create chunks of HTML and CSS that were isolated from scripts and styles, effectively invisible to developers but not to users. When it was codified into web components, this ability got a name: the shadow DOM.
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A shadow DOM is a document fragment that's attached to a host element. It becomes the visible contents of that element, and it uses the same DOM APIs as any other document. But special rules govern the boundary between shadow and what we can now think of as the "light" DOM.
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That said, my experience is that a large and complex shadow DOM is usually something to be avoided. Every shadow boundary creates additional complexity for managing focus, event listeners, DOM manipulation, and even inspecting components with the dev tools. The ideal is to keep the shadow to a minimum — use it for UI controls and decoration — but leave as much of the page in the light as possible. In the next chapter, we'll see how to make that a reality.